Color Memory Game
Vision screening

Eye test

Three checks you can run right now: a Snellen-style acuity chart, an astigmatism fan, and a red-green focus test. The acuity chart is scaled to your actual screen rather than guessed at, because you calibrate it against a bank card first. Without that step every online eye chart is showing you letters of unknown size, which makes the 20/20 label on them meaningless.

This is a screening tool. It will tell you whether your distance vision is roughly where it should be and whether one eye is noticeably worse than the other, which is the most common thing people miss about their own sight. It cannot check the health of your eye, and it cannot write you a prescription.

Step one

Calibrate your screen

Hold a bank card, driving licence, or ID card flat against the screen, upright, and drag the slider until the bar is exactly as tall as the card’s short edge. Every card of that type is 53.98mm on that side, so this tells the test how big a millimetre is on your display. Without it the letter sizes mean nothing.

53.98mm

Line the card up with the top edge of the bar and slide until the bottom edge meets the other side of the card.

96 pixels per inch
How far are you sitting from the screen?

Measure it if you can. Two metres is roughly three long paces back from a laptop on a desk. Further is better: at one metre the smallest rows fall below what most screens can render honestly.

Step two

Visual acuity chart

Cover one eye without pressing on it, read down as far as you can, and tap the smallest row you can read correctly. Then repeat with the other eye. Keep your glasses or lenses on if you normally wear them: this checks your corrected vision, which is the number that matters day to day.

A chart on a screen is not a substitute for a sight test. Real charts are printed at a known size under controlled lighting, the distance is measured rather than estimated, and an optometrist is also checking eye pressure and retinal health, which no online test can do.

Step three

Astigmatism fan

Cover one eye and look at the centre of the wheel from about an arm’s length. All the spokes should look equally black and equally sharp. If some directions look darker, thicker, or crisper than others, that uneven blur is the classic sign of astigmatism, where the cornea is curved more steeply along one axis than another.

Step four

Red and green focus check

The eye does not focus every wavelength at the same depth. Red light lands slightly behind green, which optometrists exploit with this test. Cover one eye and compare the two panels: the letters should look about equally sharp on both. Which side is crisper says something about whether your focus is sitting in front of or behind the retina.

N C H
R K D

This one is the least reliable of the three on a consumer display, because the clinical version depends on two specific narrow-band filters. Treat a clear difference as a prompt to book a test rather than as a measurement.

Reading the chart

What the numbers on an eye test chart mean

Herman Snellen designed the chart in 1862 around a single idea: define a letter by the angle it subtends at the eye rather than by its printed size. A letter on the 20/20 line is built so the whole character covers five arcminutes of your visual field, with each stroke and each gap covering one. Scale that angle up or down and you get every other line on the chart.

That is why distance has to be part of the measurement. A 20/20 letter is 8.7mm tall at six metres and 2.9mm tall at two metres, and the same letter tells you nothing if you do not know which distance it was read from. It is also why this page asks you to calibrate: the browser knows how many pixels it drew, but not how wide those pixels are on your particular panel.

The letters themselves are not arbitrary either. Modern charts use the ten Sloan optotypes, C D H K N O R S V Z, chosen because they are close to equally difficult to identify. Older charts used whatever letters were to hand, which meant an L was much easier than a G and the line you reached depended partly on your luck.

One number matters more than the absolute score: the difference between your two eyes. Most people never test them separately and have no idea that one is carrying the other. A gap of two lines or more is worth mentioning to an optometrist even if the better eye reads 20/20.

Getting it right

How to take an eye test at home properly

Measure the distance rather than pacing it. A metre estimated by eye is routinely out by twenty percent, and that error transfers straight into your result. A tape measure or a length of string cut once and reused is enough.

Get the light right. The screen should be the brightest thing you are looking at, but the room should not be dark, because a wide pupil in a dark room reduces depth of focus and makes acuity look worse than it is. Even, indirect room light with no glare or reflection on the panel is what you want.

Cover the eye you are not testing with your palm held slightly away from your face, not pressed against it. Pressing on a closed eye deforms the cornea and the vision in it stays blurry for a minute or two afterwards, which will spoil the second half of your test.

Do not squint, and do not guess the letters you already saw on the previous run. Squinting acts as a pinhole and genuinely improves acuity by a line or two, so it will hand you a result you cannot reproduce anywhere else. If you retake the test, reload the page so the letters change.

Other tests

Colour vision is a separate question

Acuity and colour vision measure different things, and someone with a red-green deficiency will usually read a Snellen chart straight down to the bottom. If that is what you came for, the colour blind test runs ten Ishihara-style plates and gives you a plate-by-plate breakdown.

For colour discrimination rather than deficiency, the hue sort asks you to arrange shuffled chips into a smooth gradient, which is the arrangement-test approach used clinically to catch small errors that plates miss. The spot the difference game is the same skill under time pressure, and how many colours the human eye can see covers where the limits actually sit.

FAQ

Eye test questions

How accurate is an online eye test?+
Accurate enough to tell you whether something has changed, and not accurate enough to prescribe from. The two things that break an online chart are unknown screen size and unknown viewing distance, and the calibration step here fixes the first one. What it cannot fix is that a real sight test also measures eye pressure, checks the retina and optic nerve, and catches conditions that have nothing to do with how small a letter you can read.
What does 20/20 vision actually mean?+
It means you can read at 20 feet what a person with normal vision reads at 20 feet. It is a reference point, not a maximum: 20/16 and 20/12 exist and are common in young adults. Outside the United States the same measure is written in metres, so 20/20 becomes 6/6. A result of 20/40 means you need to be at 20 feet to read what a normally sighted person reads at 40.
Why does the test need my bank card?+
Because a browser has no reliable way to know the physical size of your screen. CSS pixels are not a fixed length, and reported screen dimensions are wrong often enough to be useless for this. Every ID-1 card, which covers bank cards and most driving licences, is 85.60mm wide by international standard, so matching the outline to a card you already own converts screen pixels into millimetres.
Should I wear my glasses for the test?+
Yes, if you normally wear them for distance. This measures your corrected acuity, which is the number that describes how you actually see day to day and the one worth tracking. If you want to know your uncorrected acuity out of curiosity, run it a second time without them, but the corrected figure is the useful one.
What is the astigmatism fan testing?+
Whether your cornea is curved evenly. A perfectly spherical cornea focuses light from every direction at the same depth, so all the spokes look equally sharp. If the cornea is steeper along one axis, light from that direction focuses at a different depth, and the spokes running that way look darker or crisper than the rest. It is very common, it is fully correctable, and the axis of the darkest spokes is exactly what goes on a prescription.
How often should I get my eyes tested?+
Every two years is the usual advice for adults with no symptoms and no risk factors, and annually if you are over 60, diabetic, have a strong family history of glaucoma, or already wear a strong correction. Children should be screened before starting school. Any sudden change, flashes, floaters, or loss of part of your visual field is a same-day matter rather than a two-year one.
Can this test detect colour blindness?+
No. Acuity and colour vision are separate things, and someone with a colour vision deficiency will usually read a Snellen chart perfectly. For that you want a plate test, which is a different tool on this site.